Seismic behavior of geosynthetic encased columns and ordinary stone columns

This study is concerned with evaluating and comparing the behavior of geosynthetic encased stone columns (GECs) and ordinary (conventional) stone columns (OSCs) during and after seismic excitations. For this purpose, well instrumented GECs and OSCs are installed in kaolinite clay beds consolidated in a large steel tank. In order to simulate the seismic behavior of columns supporting an embankment, surcharge loads are applied and the experimental setup is subjected to large-scale shaking table tests. The strains in the encasement are measured by making use of water-proof strain gauges during the course of the experiments. The vertical load capacities of GECs and OSCs after the seismic excitation were measured by a series of stress controlled column load tests. The experimental data at hand suggests that under the action of seismic loads there is a significant strain demand on the encasement confining the GECs. An almost linear relationship between the seismic energy input expressed in terms of IA (Arias Intensity) and reinforcement strain amplitude is observed. GECs in general have exhibited a superior performance both under static and seismic loads when compared to OSCs.     

On the shear failure mode of granular column embedded unit cells subjected to static and cyclic shear loads

Since the initial conception of geosynthetic encased columns (GECs), exhaustion of column capacity due to vertical loads in bulging and punching failure modes were readily recognized. This lead to a vast majority of the available research on GECs to be about the behavior of columns under the action of vertical loads. Recently, two other likely and perhaps more dominant failure modes for granular columns namely, shear and bending failure modes, were identified. The purpose of this paper is to study the behavior of unit cells containing ordinary stone columns (OSCs) and GECs under static and cyclic lateral loads where shear failure of the column is imminent. 1-g physical tests are conducted with a novel apparatus, designated as Unit Cell Shear Device (UCSD), to model the behavior of the unit cells located close to the toe of an embankment where OSCs and GECs experience significant lateral loading. Overall failure envelope and strength parameters for GECs with varying reinforcement stiffnesses are quantified under static and cyclic lateral loading conditions. The distribution and magnitude of reinforcement strains in horizontal (hoop) and vertical direction of the columns are also considered.     

竖向土工加筋体对碎石桩承载变形影响的模型试验研究

在碎石桩桩顶一定深度内包裹竖向土工加筋体形成筋箍碎石桩,能有效提高碎石桩的承载能力,控制复合地基沉降量。采用分级加载方式,设计并完成了两组较大比例室内模型试验,对比分析了筋箍碎石桩和传统碎石桩的承载变形特性,进而探讨了筋箍碎石桩的加筋机理和鼓胀变形模式,重点分析了竖向土工加筋体的应力应变特征。分析结果表明：竖向土工加筋体能有效约束碎石桩的侧向鼓胀,在微小侧向变形内提供足够的径向约束应力;筋箍碎石桩的最大鼓胀变形多发生于加筋体以下区域,其破坏模式与筋体材料、桩体、桩周土体及其相互作用和协调变形密切相关;筋箍碎石桩的桩顶和桩底桩土应力比均明显大于传统碎石桩,上部土工加筋体在提高桩体刚度的同时,可有效地将上部荷载传递至桩底较好土层。     

Centrifuge modeling of geosynthetic-encased stone column-supported embankment over soft clay

Geosynthetic-encased stone column (GESC) has been proven as an effective alternative to reinforcing soft soils. In this paper, a series of centrifuge model tests were conducted to investigate the performance of GESC-supported embankment over soft clay by varying the stiffness of encasement material. The enhancement in the performance of stone columns encased with geosynthetic materials was quantified by comparing the test with ordinary stone columns (OSCs) under identical test conditions. The test results reveal that by encasing stone columns with geosynthetic material, a significant reduction in the ground settlement, relatively faster dissipation of excess pore pressure and enhanced stress concentration ratio was noticed. Moreover, with the increase in the encasement stiffness from 450 kN/m to 3300 kN/m, the stress concentration ratio increased from 4 to 6.5, which signifies the importance of encasement stiffness. In addition, a relatively lower value of soil arching ratio observed for GESCs compared to OSCs indicate the formation of a relatively strong soil arch in the GESC-supported embankment. Interestingly, under embankment loading, GESCs fail by bending while OSCs fail by bulging. The stress reduction method can be used to calculate the settlement of GESC-supported embankment with larger stress reduction factor than that in the OSC-supported embankment. Finally, the limitation of the construction of the embankment at 1 g was addressed.     

Centrifuge model tests on the deformation behavior of geosynthetic-encased stone column supported embankment under undrained condition

A series of centrifuge model tests were carried out to investigate the performance of geosynthetic-encased stone columns (GESCs) supported embankment under undrained condition. The influence of stiffness of encasement, basal reinforcement and embankment loading on the deformation behavior of GESCs were also assessed. The centrifuge test results reveal that under undrained condition, compared to ordinary stone column (OSC) supported embankment, the settlement of column has reduced by 50% and 34% when columns were encased with high and low stiffness geogrids respectively. Moreover, under identical embankment loading condition, the stress concentration ratio has increased significantly upon inclusion of basal reinforcement in the GESCs supported embankment. In case of OSCs supported embankment, columns experiences bulging in the top portion, inward bending in the central portion and a noticeable shear at the bottom portion. However, when columns were encased with geogrid layer, bulging in the top portion was significantly reduced but the inward bending of columns were noticed. With the inclusion of basal reinforcement, bending curvature of columns increases thereby inducing higher settlement in columns and relatively lesser settlement in surrounding soil. The differential settlement between the encased column and the surrounding soil under embankment loading has been considerably reduced with the inclusion of basal reinforcement.     

Geosynthetic-encased stone columns: Numerical evaluation

Stone columns (or granular piles) are increasingly being used for ground improvement, particularly for flexible structures such as road embankments, oil storage tanks, etc. When the stone columns are installed in extremely soft soils, the lateral confinement offered by the surrounding soil may not be adequate to form the stone column. Consequently, the stone columns installed in such soils will not be able to develop the required load-bearing capacity. In such soils, the required lateral confinement can be induced by encasing the stone columns with a suitable geosynthetic. The encasement, besides increasing the strength and stiffness of the stone column, prevents the lateral squeezing of stones when the column is installed even in extremely soft soils, thus enabling quicker and more economical installation. This paper investigates the qualitative and quantitative improvement in load capacity of the stone column by encasement through a comprehensive parametric study using the finite element analysis. It is found from the analyses that the encased stone columns have much higher load carrying capacities and undergo lesser compressions and lesser lateral bulging as compared to conventional stone columns. The results have shown that the lateral confining stresses developed in the stone columns are higher with encasement. The encasement at the top portion of the stone column up to twice the diameter of the column is found to be adequate in improving its load carrying capacity. As the stiffness of the encasement increases, the lateral stresses transferred to the surrounding soil are found to decrease. This phenomenon makes the load capacity of encased columns less dependent on the strength of the surrounding soil as compared to the ordinary stone columns.     

Uniaxial compression behavior of geotextile encased stone columns

The bearing capacity and failure mechanism of encased stone columns are affected by many factors such as encasement length, relative density, strength and stiffness of the encasement material. In soft soils where surrounding soil pressure is low, especially in the top section, the stone columns may be close to a uniaxial compression state, where the uniaxial compression strength controls the bearing capacity of the stone columns. A series of large-scale triaxial tests on ordinary stone columns and uniaxial tests on geotextile encased stone columns have been performed. The stone columns were 300?mm in diameter and 600?mm in height. Samples of four different relative densities, and five types of geotextiles were used in the tests to study the effect of initial void ratio and encasing materials on the uniaxial compression behavior of the stone columns. The results show the uniaxial compressive strength of the encased stone columns is not affected by the initial void ratio but mainly by the tensile strength of the encasing geotextiles. The stress strain curves of the encased stone columns under uniaxial loading condition are nearly liner before failure, which is similar to the tensile behavior of the geotextiles.     

Laboratory investigation of the behavior of a geosynthetic encased stone column in sand under cyclic loading

This paper presents the results of a laboratory investigation into the behavior of a geosynthetic encased stone column (GESC) installed in sand under cyclic loading using a reduced-scale model. A number of test variables were considered, such as the geosynthetic encasement stiffness and the cyclic loading characteristics, including loading frequency and amplitude. The results indicate among other things that the overall benefit of the encasement to the performance of the stone column is greater under cyclic loading than under static loading. It is shown that the degree of load transfer to the column becomes smaller when subjected to cyclic loading than under static loading, leading to a 25% decreased stress concentration ratio. The encasement is found to be more effective in improving the stone column performance when subjected to lower frequency and/or smaller amplitude loading. The lateral bulging zone of the GESC under cyclic loading tends to extend beyond the reported critical encasement length for an isolated static loading case, and therefore full encasement is recommended. Practical implications of the findings are discussed in detail.     

Short-term and long-term behavior of geosynthetic-reinforced stone columns

Stone columns are often used to improve the load-carrying characteristics of weak soils. In very soft soils, however, the bearing capacity of stone columns may not significantly improve the load-carrying characteristics due to the very low confinement of the surrounding soil. In such cases, encased stone columns (ESCs) or horizontally reinforced stone columns (HRSCs) may be used. Although ESCs have been studied extensively, few studies have been done on HRSCs. In addition, very limited studies are available on ESCs and HRSCs under the same conditions. Moreover, no studies have been carried out to compare the long-term and short-term behavior of HRSCs with that of ESCs. In this research, therefore, numerical analyses are performed on various types of reinforced end-bearing stone columns to compare their behavior under both long-term and short-term conditions under various loading conditions. The Advanced Modified Cam-clay model for clay and the Hardening Soil model for stone column materials are used. The results show that with proper reinforcing stone columns, in addition to a considerable reduction in settlement, the consolidation time can be greatly decreased and most of the settlement will occur during the loading period. Also, the consolidation settlement rate may be increased by using a smaller column diameter and a larger area replacement ratio for the unit cell, stiffer geosynthetic reinforcements, and greater values for the internal friction angle of the stone column materials.     

Consolidation of soft clay foundation improved by geosynthetic-reinforced granular columns: Numerical evaluation

Soft clays are problematic soils as they present high compressibility and low shear strength. There are several methods for improving in situ conditions of soft clays. Based on the geotechnical problem's geometry and characteristics, the in situ conditions may require reinforcement to restrain instability and construction settlements. Granular columns reinforced by geosynthetic material are widely used to reduce settlements of embankments on soft clays. They also accelerate the consolidation rate by reducing the drainage path's length and increasing the foundation soil's bearing capacity. In this study, the performance of encased and layered granular columns in soft clay is investigated and discussed. The numerical results show the significance of geosynthetic stiffness and the column length on the embankment settlements. Furthermore, the results show that granular columns may play an important role in dissipating the excess pore water pressures and accelerating the consolidation settlements of embankments on soft clays.     

Vertical cyclic loading response of geosynthetic-encased stone column in soft clay

Dynamic responses of the geosynthetic-encased stone column (GESC) supported embankment under traffic loads have become a hot topic. This study investigates the responses of GESC improved ground under vertical cyclic loading. A series of laboratory tests in a designed model test tank have been carried out with different loading parameters (varied loading amplitudes and frequencies), different column dimensions (varied encasement lengths and column diameters). In the tests, the soil-column stress distribution, accumulated settlement of loading plate, excess pore water pressure in the surrounding soil and lateral bulging of the stone column are monitored. Experimental results indicate that the vertical stress on the stone column increases with the increment of encasement length, and decreases with the increment of column diameter, loading amplitude and loading frequency. The increasing stress on the surrounding soil leads to a greater accumulated settlement of the loading plate and excess pore water pressure, while the increasing stress on the column leads to larger lateral bulging of the column. Excess pore water pressure dissipates effectively through vertical and horizontal drainage channels provided by the stone column and the sand bed. The geosynthetic encasement prevents the clay from obstructing the drainage channel by filtration and guarantees the drainage effect.     

A numerical analysis of a fully penetrated encased granular column

This paper studies the performance of an individual encased granular column that is embedded in soft soil using a numerical analysis. The numerical analysis is verified by experimental tests that are performed in the laboratory, using a model encased sand column that is embedded in a soft clay deposit. In addition to bearing stress-settlement response, detailed characterizations of the encased column, in terms of the distribution of lateral earth and sleeve-induced pressure along the column length, are determined. The numerically analyzed results are compared with those for the model tests and analytical results. Parametric studies over the encasement stiffness, the diameter of the granular column and the loading area are conducted to determine the influence of encasement on the column. The sleeve-induced confining pressure and the bearing stress of the encased sand columns, calculated using the cavity expansion theory and the simplified approach that assumes a constant volume for the granular column, are compared with the numerical results to justify the use of these two methods. The numerical results show that the stiffness of the encasement significantly affects the bulging length of an encased granular column. An increase in the column diameter or the loading area produces a significant reduction in the sleeve-induced confining pressure, which leads to a reduction in the bearing stress improvement of an encased granular column, but the total load supported by the loading plate has an almost linear relationship with the loading plate diameter/column diameter ratio.     

Effect of basal reinforcement on performance of floating geosynthetic encased stone column-supported embankment

In this paper, two centrifuge modeling tests were performed to investigate the influence of basal reinforcement on the global performance of floating geosynthetic encased stone column (GESC)-supported embankments. Based on the centrifuge tests, a 3-dimensional (3D) numerical modeling was carried out to investigate the influence of basal reinforcement on the deformation behavior of the floating GESC-supported embankment. The centrifuge and numerical modeling results showed that the basal reinforcement reduced total and uneven settlement at the embankment crest and base significantly. Moreover, the inclusion of the basal reinforcement significantly reduced the lateral displacement on top of the column, preventing outward bending of the floating GESCs below the embankment toe. However, the basal reinforcement increased the lateral displacement at the bottom of columns.     

Laboratory model studies on unreinforced and geogrid-reinforced sand bed over stone column-improved soft clay

Results from a series of laboratory model tests on unreinforced and geogrid-reinforced sand bed resting on stone column-improved soft clay have been presented. The diameter of stone column and footing has been taken as 50 mm and 100 mm, respectively for all the model tests carried out. Load was applied to the soil bed through the footing until the total settlement reached at least 20% of footing diameter. As compared to unimproved soft clay, the increase in load-carrying capacity under different improved ground conditions has been observed. Influences of the thickness of unreinforced as well as geogrid-reinforced sand bed and the size of geogrid reinforcement on the performance of stone column-improved soft clay bed have also been investigated. Significant improvement in load-carrying capacity of soft soil is observed due to the placement of sand bed over stone column-improved soft clay. The inclusion of geogrid layer within sand bed further increases the load-carrying capacity and decreases the settlement of the soil. Due to the placement of sand bed, the bulge diameter of stone column reduces while the depth of bulge increases. Further reduction in the bulge diameter and increase in bulge depth are observed due to application of geogrid layer. The optimum thickness of unreinforced sand bed is twice the optimum thickness of geogrid-reinforced sand bed. Under specific material properties and test conditions, it is further observed that the optimum diameter of geogrid layer is thrice the diameter of footing.     

Bearing capacity of geogrid reinforced sand over encased stone columns in soft clay

Stone columns develop their load carrying capacity from the circumferential confinement provided by the surrounding soils. In very soft soils, the circumferential confinement offered by the surrounding soft soil may not be sufficient to develop the required load carrying capacity. Hence a vertical confinement would yield a better result. The load carrying capacity is further increased with the addition of a sand bed over the stone columns. In the present study, a series of laboratory model tests on an unreinforced sand bed (USB) and a geogrid-reinforced sand bed (GRSB) placed over a group of vertically encased stone columns (VESC) floating in soft clay and their numerical simulations were conducted. Three-dimensional numerical simulations were performed using a finite element package ABAQUS 6.12. In the finite element analysis, geogrid and geotextile were modeled as an elasto-plastic material. As compared to unreinforced clay bed, an 8.45 fold increase in bearing capacity was observed with the provision of a GRSB over VESC. The optimum thickness of USB and GRSB was found to be 0.2 times and 0.15 times the diameter of the footing. A considerable decrease in bulging of columns was also noticed with the provision of a GRSB over VESC. Both the improvement factor and stress concentration ratio of VESC with GRSB showed an increasing trend with an increase in the settlement. It was observed that the optimum length of stone columns and the optimum depth of encasement of the group of floating VESC with GRSB are 6 times and about 3 times the diameter of the column respectively.     

3-Dimensional numerical modeling of geosynthetic-encased granular columns

In this paper, series of three-dimensional (3-d) numerical modeling of geosynthetic-encased granular columns were performed both in model and prototype scale using FLAC^3D software to understand the lateral load carrying capacity of ordinary and geosynthetic encased granular columns (OGC and EGC). In the first part of the study, numerical modeling of direct shear tests were carried out. The soil in the direct shear box was reinforced with two different diameters of granular columns (50 mm and 100 mm) and three different patterns of arrangement (single, triangular and square) to study the effect of group confinement. The numerical simulations were carried out at four different confining pressures namely 15, 30, 45 and 75 kPa. From the numerical simulations it was observed that higher shear stresses are mobilized inside the granular column due to geosynthetic encasement and the magnitude of shear stress increases with increase in the normal pressure. It was found that the tensile forces in the geosynthetic encasement were mobilized both in circumferential and vertical directions, which helps in mobilizing additional confinement in the granular column. In the second part, the influence of the geosynthetic encasement of granular column treated soft ground was demonstrated through 3-dimensional slope stability analyses.     

Numerical evaluation of a granular column reinforced by geosynthetics using encasement and laminated disks

Structures built on soft strata may experience substantial settlement, large lateral deformation of the soft layer and global or local instability. Granular columns reinforced by geosynthetic materials reduce settlement and increase the bearing capacity of the composite ground. Reinforcement is more common in the form of geosynthetic encasement, but laminated disks can also be used. This paper compares these two forms of reinforcement by means of unit cell finite element analyses. Numerical results were initially validated using field and experimental data, and parametric studies were subsequently performed. The parametric studies varied the geosynthetic interval and the geosynthetic tensile stiffness of the laminated disks as well as the length of the reinforced column. The analyses showed that in both modes; encasement and laminated disks; the geosynthetic increases the vertical stress mobilized on the reinforced column and reduces settlement on soft soil. It was also observed that in order to achieve the same performance as with encased column, the optimum interval between laminated disks is dependent on the stiffness of the geosynthetics and the column reinforced length.     

Comparative Study on the Behavior of Encased Stone Column and Conventional Stone Column

Stone columns, one of the most commonly used soil improvement techniques, have been utilized worldwide to increase bearing capacity and reduce total and differential settlements of structures constructed on soft clay. Stone columns also act as vertical drains, thus speeding up the process of consolidation. However, the settlement of stabilised bed is not reduced in many situations for want of adequate lateral restraint. Encasing the stone column with a geogrid enhances the bearing capacity and reduces the settlement drastically without compromising its effect as a drain, unlike a pile. The behavior of the encased stone column stabilized bed is experimentally investigated and analysed numerically. In the numerical analysis, material behaviour is simulated using Soft Soil, Mohr Coulomb and Geogrid models for clay, stone material and encasement respectively and is validated with experimental results. The parametric study carried out on varying the L/D ratio (L = length of the column; D = diameter of the column) of column, stiffness of geogrid and angle of internal friction of stone material gives a better understanding of the physical performance of the encased stone column stabilized clay bed.     

Experimental study on the load bearing behavior of geosynthetic reinforced soil bridge abutments on yielding foundation

This paper presents an experimental study of the load bearing behavior of geosynthetic reinforced soil (GRS) bridge abutments constructed on yielding clay foundation. The effects of two different ground improvement methods for the yielding clay foundation, including reinforced soil foundation and stone column foundation, were evaluated. The clay foundation was prepared using kaolin and consolidated to reach desired shear strength. The 1/5-scale GRS abutment models with a height of 0.8 m were constructed using sand backfill, geogrid reinforcement, and modular block facing. For the GRS abutments on three different yielding foundations, the reinforced soil zone had relatively uniform settlement and behaved like a composite due to the higher stiffness than the foundation layers. The wall facing moved outward with significant movements near the bottom of facing, and the foundation soil in front of facing showed obvious uplifting movements. The vertical stresses transferred from the footing load within the GRS abutment and on the foundation soil are higher for stiffer foundation. The improvement of foundation soil using geosynthetic reinforced soil and stone columns could reduce the deformations of GRS abutments on yielding foundation. Results from this study provide insights on the practical applications of GRS abutments on yielding foundation.     

Performance of geosynthetic-encased stone column-improved soft clay under vertical cyclic loading

This paper presents the results of a laboratory investigation into the performance of geosynthetic-encased stone column-improved (GESC-improved) soft clay under vertical cyclic loading. A reduced-scale model is adopted to perform a series of tests considering the principal parameters, such as the cyclic loading characteristics, including the loading frequency and amplitude, and the encasement length. The results indicate that, among other things, the overall benefit of the geosynthetic encasement of stone columns installed in soft clay is greater under cyclic loading than under static loading, and that the cyclic effect tends to lead to a stress concentration ratio that is smaller than that under static loading. The effectiveness of this encasement in improving the performance of GESCs becomes greater when subjected to cyclic loading with a lower loading frequency and/or a smaller amplitude. The settlement and pore pressure variations with the encasement length, together with the exhumed GESCs taken after the tests, suggest that full encasement is necessary to maximize the performance of GESCs under cyclic loading.